Artificial lighting systems for closed environments often aim at improving the visual comfort experienced by users. For example, lighting units are known for simulating natural lighting, specifically sunlight illumination, that provide dichroic light to be emitted from a dichroic light exiting surface, where the dichroic light comprises a directional light portion of direct light having a first (lower) correlated color temperature (CCT) and a diffused light portion of diffused light having a second (larger) CCT.
Exemplary embodiments of such lighting systems using, for example, Rayleigh-like diffusing layers are disclosed in several applications such as WO 2009/156347 A1, WO 2009/156348 A1, WO 2014/076656 A1, and WO 2015/172821 A1 filed by the same applicants. The therein disclosed lighting systems use, for example, a light source producing visible light, and a panel containing nanoparticles used in transmission or reflection. During operation of those lighting systems, the panel receives the light from the light source and acts as a so-called Rayleigh diffuser, namely it diffuses incident light similarly to the earth atmosphere in clear-sky conditions.
In further embodiments such as disclosed in WO 2014/075721 A1, the unpublished international patent application PCT/EP2015/077169, and the not yet published international patent application PCT/EP2015/069790 filed by the same applicants on 28 Aug. 2016, the concepts of direct light with lower CCT and diffused light with larger CCT are implemented exemplarily in a linearly extending and in a compact configuration of lighting systems.
As mentioned, the implementations referred to above use nanoparticles that interact with light, due to their nanosize, in the Rayleigh (or Rayleigh-like) scattering regime and are embedded in a host material (surrounding matrix). It is well known from fundaments of light-scattering that transparent nanoparticles having different refraction index with respect to the matrix, and having sizes (significantly) smaller than visible wavelength, will preferentially scatter the blue part of the spectrum, and transmit the red part. Specifically, the single particle scattering cross-section is given by
      σ    ⁡          (      λ      )        =            2      3        ⁢          π      5        ⁢                  D        6                              n          h          2                ⁢                  λ          4                      ⁢                  (                                            m              2                        -            1                                              m              2                        +            2                          )            2      and an ensemble scattering cross-section amount is given byσ(λ)tot=N·σ(λ),with N being the number of nanoparticles per unit area (see below).
The optical parameters of the scattering are defined by the size and refractive index of the nanoparticles as well as the number of particles distributed in, for example, a transparent matrix and by the refractive index of that matrix. For nanoparticles, the Rayleigh scattering process depends on three parameters D, m, and N as summarized in the following:
D relates to the size d of the nanoparticles. Specifically, an effective particle diameter D=d nh is considered, where d [meter] is the average particle size over the particles distribution in the case of spherical particles, and as the average thickness of the particles in an assigned propagation direction in the case of non-spherical particles. While the wavelength-dependence of the scattering efficiency per single particle approaches the λ−4 Rayleigh-limit law for particle sizes smaller or about equal to 1/10 of the wavelength λ, a respective acceptable optical effect may be reached already in the above range for the size of the nanoparticles, often referred to as Rayleigh-like scattering. On the other side, the scattering efficiency per single particle, proportional to D6, decreases with decreasing particle size, thereby making the use of too small particles inconvenient and requiring a high number of particles in the propagation direction, which in turn may be limited by an achievable filling-fraction.
m relates to the index mismatch of the nanoparticles and the matrix. Specifically, the chromatic effect is based on nanoparticles having a refractive index that is different from the refractive index of the embedding matrix. To scatter, the nanoparticles have a real refractive index np sufficiently different from the refractive index nh of the host material in order to allow light scattering to take place. E.g., the above mentioned prior art systems use a specific solid particle within a specific host material, thereby setting the scattering condition for a fixed ratio
  m  =            n      p              n      h      between the particle and host medium refractive indexes. m is referred to as the relative index of refraction.
N relates to the number of nanoparticles involved in the scattering. Specifically, the chromatic effect is based on the number of nanoparticles per unit area seen by the impinging light propagating in a given direction as well as the volume-filling-fraction f. Specifically, the chromatic effect is based on the number N of nanoparticles per unit area that are e.g. embedded in the chromatic diffusive layer.
Herein, a system and a method for Rayleigh-like scattering is disclosed that can be used, in particular, for implementation in a lighting system that imitates the sun-sky illumination for varying conditions.
Thus, the present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.